Three really good questions.
1. I don't think that threshold's influence on IG is about separation. When IG is too low (or too high) I have seen CPU QBs bypass WIDE OPEN receivers. I really think that it is something 'under the hood' that causes this affect and it isn't something that manifests itself on the field. Food for thought: on NCAA 14, nearly EVERY gameplay slider and every penalty slider affects IG (we raised RTK once by 5 points and had to adjust IG downwards). Fortunately, I have not seen this here.
WRC: I believe that this influences nearly every facet of the WRs game, not just how well they catch....I'll get back to this in a moment....
OPI: I prefer it high as it induces better catch animations from WRs and they are more aggressive going for the catch when WRC is at or below 50. If you want to go really low, WRs will not go after catches...as such, you would need to raise WRC to compensate.
PCV: Remember that high PCV does not equal
better coverage. I think of it as more of the slider which influences how quickly a DB reacts to a ball...which is why you will see more deflections at higher settings. Problem: DBs will begin to completely ignore the guys they are supposed to be covering in man coverage.
DPI: The higher it is, the more tightly the DBs cover. Pure and simple.
OPI/DPI ratio: Whichever number is higher has the advantage. So, if you want to give the DB the advantage, their number should be higher...however, you might be better served also raising WRC as well. Remember, this is universal, so tweaks would have to be made to both sides.
You answered your own question....it makes a difference. Could you potentially manipulate sliders by having different main menu and CFM sliders? Sure, if we had 3 years and this was the last version of Madden ever made. Without stating up-front that they have to match, we would get people with default main menu seeing different things then other people who had them both matching. It's the same mess we were in with the "Cam Newton" difficulty before King figured that out....some people had it set to All Pro (whining, "It's too easy...") and others had it set to AM (whining, "It's too hard...").